[SCSI] fix panic when ejecting ieee1394 ipod

The scsi_library routines don't correctly set DMA_NONE when
req->data_len is zero (instead they check the command type first, so
if it's write, we end up with req->data_len == 0 and direction as
DMA_TO_DEVICE which confuses some drivers)

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>